say the truth or not?

just wondering, if saying the truth to a person is a bad thing to do?or not? i mean, you will say this thing to a person because you are just concern to her, but you know also that she will feel bad if you will say it to her and your frienship will also be affected. but if you'll just keep it to yourself, your conscience will just bugging you always and you may also regret it after.
do you have that courage to say or you just keep it, until someone will tell it to her?